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The sample verification of anti metal magnetic separators usually uses aluminum or stainless steel plates as the metal substrate. Aluminum plates are non ferromagnetic metals, while the choice of stainless steel plates often depends on what is at hand -304 stainless steel is non ferromagnetic, 430 stainless steel and carbon steel are ferromagnetic, but they may not be distinguished when purchasing. If both substrates pass through, the scheme is considered "universal for metals".
This' universal 'assumption often fails when encountering ferromagnetic metal substrates. The same magnetic isolation sheet can restore good reading distance when attached to an aluminum plate, but when attached to a carbon steel shelf or ferromagnetic stainless steel shell, the reading distance cannot be restored, and it is even worse than not attaching a magnetic isolation sheet. The problem is not with the permeability of the magnetic separator, but with the fact that aluminum and carbon steel have opposite attitudes towards magnetic flux.
Non ferromagnetic metals: eddy currents are the only enemy
The magnetic permeability of aluminum, copper, and non ferromagnetic stainless steel (such as 304) is close to 1, the same as that of air. They only respond to alternating magnetic fields in one way: by inducing eddy currents. The reverse magnetic field of eddy currents cancels out the incident magnetic flux, weakening the effective magnetic field obtained by the tag antenna.
Bypass suppression vortex mechanism
The working mechanism of the magnetic separator on this substrate is clear: it provides a high magnetic permeability bypass, and magnetic field lines preferentially propagate along the magnetic separator, bypassing the eddy current zone on the metal surface. The higher the μ 'of the magnetic separator and the larger the coverage area, the stronger the bypass effect, and the more thoroughly the eddy current is suppressed. The verification data on the aluminum plate describes the effect of this "bypass suppression vortex" mechanism.
Ferromagnetic metal: The substrate itself is a 'flux competitor'
The magnetic permeability of ferromagnetic materials such as carbon steel, 430 stainless steel, and galvanized steel plates ranges from hundreds to thousands of levels, far higher than that of air. When the label is attached to this substrate, the magnetic field lines face not a choice between "going through metal or air", but a competition between "going through magnetic separators or ferromagnetic substrates".
Redistribution of magnetic flux between labels, separators, and ferromagnetic substrates
The magnetic permeability of a ferromagnetic substrate may be on the same level or even higher than the effective magnetic permeability of the magnetic separator. The magnetic field lines are redistributed between the tag antenna, the magnetic separator, and the ferromagnetic substrate: a portion of the magnetic flux is guided by the magnetic separator, while another portion is "sucked" into the ferromagnetic substrate. The magnetic flux absorbed by the ferromagnetic substrate forms a magnetic circuit inside the substrate, changing the magnetic field topology around the antenna, and causing the equivalent inductance and resonant frequency of the tag antenna to shift accordingly. The role of magnetic separators on ferromagnetic substrates has shifted from being the "sole flux guide" to a "competitor competing with the substrate for magnetic flux". Its effective magnetic permeability is no longer determined by the material itself, but by the relative relationship between the magnetic separator μ 'and the substrate μ'. If the μ 'of the substrate is higher than the effective μ' of the magnetic separator, more magnetic flux will pass through the substrate, and the guiding effect of the magnetic separator will be weakened or even completely ineffective.
The second failure path on a ferromagnetic substrate: the saturated 'position' has changed
On non ferromagnetic metals, saturation of the magnetic separator means that magnetic flux penetrates the separator and enters the metal, causing an increase in eddy current losses. Saturation is a problem with the magnetic separator itself.
On ferromagnetic substrates, saturation may occur first within the substrate. The saturation magnetization of ferromagnetic materials is usually in the range of 1.0-2.0 T, which is higher than that of ferrites but lower than that of nanocrystals. When the magnetic flux density near the tag antenna is high, the local area of the ferromagnetic substrate may enter saturation first, causing a sharp decrease in magnetic permeability and a change in magnetic flux distribution. The saturation of the substrate in turn changes the working point of the magnetic separator - the magnetic flux originally "shared" by the substrate returns to the separator, and the local magnetic flux density of the separator increases, which may follow saturation after the substrate is saturated. The superposition of two saturation paths makes it more difficult to predict the working point of the magnetic separator on the ferromagnetic substrate than on the aluminum plate. The saturation on the aluminum plate depends only on the Bs and thickness of the magnetic separator itself; The saturation on a ferromagnetic substrate depends on the Bs, thickness, and relative area of both the spacer and the substrate, which is a coupling problem.
Why does the 'universal solution' fail on ferromagnetic substrates
A magnetic separator optimized for aluminum plate, with its μ 'and thickness designed according to the logic of "bypass suppression of eddy currents". This logic does not hold on ferromagnetic substrates because:
Firstly, the distribution of magnetic flux has changed
The magnetic separator no longer monopolizes the magnetic flux path, and the substrate separates a portion of the magnetic flux. The effective μ 'of the magnetic separator is' diluted', and the actual guided magnetic flux is lower than the design expectation.
Secondly, the resonance condition has changed
The magnetic permeability of the ferromagnetic substrate participates in the magnetic circuit around the antenna, and the equivalent inductance of the antenna is different from that on an aluminum plate. The matching network adjusted on the aluminum plate may have become detuned on the ferromagnetic substrate.
Thirdly, saturation behavior has changed
The substrate may saturate before the magnetic separator, and after saturation, the magnetic flux is redistributed, causing a nonlinear change in the operating point of the magnetic separator. The design assumption of the "linear zone" on aluminum plates no longer holds true on ferromagnetic substrates.
The combination of these three changes resulted in the abnormal performance of the "aluminum plate verification passed" magnetic separator on the ferromagnetic substrate. It's not that the magnetic separator has' deteriorated ', but rather that the magnetic circuit topology it faces is completely different.
Selection judgment based on "substrate permeability x magnetic separator μ ′ x magnetic circuit competition"
Firstly, in sample verification, it is necessary to clearly distinguish between ferromagnetic and non ferromagnetic substrates and not mix them together
Aluminum plates, copper plates, and 304 stainless steel are non ferromagnetic, while carbon steel, 430 stainless steel, and galvanized steel plates are ferromagnetic. The failure mechanisms of the two types of substrates are different, and the validation data cannot be extrapolated from each other. If the actual application involves ferromagnetic substrates, sample verification must use ferromagnetic materials of the same or similar grades.
Secondly, on ferromagnetic substrates, the μ 'of the magnetic separator needs to compete with the substrate μ', and the selection logic shifts from 'higher, better' to 'matching competition'
If the substrate is low carbon steel (in the range of hundreds to thousands of μ '), the effective μ' of the magnetic separator needs to be close to or exceed the substrate μ 'in order to have an advantage in magnetic flux distribution. If the substrate is a high permeability alloy steel, the magnetic separator may need to choose a high μ 'system such as nanocrystals, or compensate for the deficiency of single point μ' by increasing the coverage area.
Thirdly, on ferromagnetic substrates, consider "substrate saturation" as an independent verification term, rather than just checking the saturation margin of the magnetic separator
Request suppliers or internal simulations to provide the magnetic permeability retention rate of ferromagnetic substrates under working magnetic flux density. If the substrate is close to saturation near the label, the design of the magnetic separator needs to be adjusted accordingly - increasing the area of the magnetic separator to share the magnetic flux, or reducing the working magnetic flux density of the antenna.
Fourthly, on a ferromagnetic substrate, antenna matching must be tuned in the combination state of the "tag magnetic isolation sheet ferromagnetic substrate", rather than tuning the antenna first and then mounting it
The magnetic permeability of the ferromagnetic substrate is involved in the magnetic circuit around the antenna, and the matching network needs to be calibrated in the complete combination state. If the antenna is first adjusted in free space and then attached to a ferromagnetic substrate, the resonance offset may exceed the chip tolerance.
Advanced Institute (Shenzhen) Technology Co., LtdThe anti metal magnetic isolation sheet product line covers ferrite composite sheets and nanocrystalline composite sheets, providing differentiated read distance recovery evaluation data on ferromagnetic and non ferromagnetic substrates, as well as recommendations for matching magnetic isolation sheet μ 'with substrate μ'.
The product supports material system matching based on the actual magnetic permeability grade of the metal substrate, helping engineers extend the "aluminum plate verification" to "ferromagnetic substrate availability". For more technical information, please visithttp://www.avanzado.cn/.